1. Field of the Invention
This invention relates to an improved method and apparatus for continuously feeding material through a substantially vertical flow treatment tower.
The invention is particularly directed toward an improved method and apparatus for continuously feeding a slurry or liquid of high viscosity into a substantially vertical upflow treatment tower so as to retain all the material passing through the tower, substantially for the same length of time in the tower.
2. Description of the Prior Art
In a continuous feed, vertical upflow tower, one or more portions of the material may flow from the lower end of the tower to the upper end in one or more streams of material, the streams moving at a faster speed through the tower than the speed at which the remaining portions of the material in the tower move through the tower. Such a condition is commonly known as "channeling". Channeling can readily occur when the tower has a central single, lower inlet and a central single, upper outlet. The material, entering the tower through a central, single, lower inlet, which inlet usually has a small diameter relative to the diameter of the tower, tends to form a central stream which meanders up through the tower at a faster rate of speed than the remaining portion of the material within the tower, which portion is generally located at the sides of the tower.
In towers used to treat material, such as, for example, by chemical, biochemical, or thermal treatment, the channeling effect results in shorter retention times for the material travelling in the streams as compared to the retention times for the material not travelling in the streams, thereby resulting in the material being unevenly treated during its passage up through the tower.
In an attempt to minimize the channeling effect, vertical upflow retention towers, such as, for example, those used in the continuous bleaching of pulp, the tower can be made tall and slim to provide a larger than normal height-to-diameter ratio. In slim towers, there is less likelihood of channeling occurring. Such towers can be satisfactorily used to minimize channeling in small capacity bleaching plants or where a minimum amount of retention time is required to treat pulp. However, towers with a high height-to-diameter ratio become very expensive when longer retention times or larger bleaching plant capacities are required. Thus, towers of a lower height-to-diameter ratio are generally used for the sake of economy, but such towers are more prone to channeling.
In upflow towers, attempts have been made to minimize the channeling effect by providing a number of agitators set at various elevations from the bottom to the top of the tower, but they have not been completely effective.
The higher the consistency or viscosity of the material flowing through the tower, the greater the possibility of channeling occuring. When using high consistency or high viscosity material in upflow towers, the towers have been provided with hemispherical or conical bottoms together with rotary mixers or flow distributors at the inlet or base of the tower. However, none of these solutions have been found to be very effective unless the tower has a high height-to-diameter ratio which, as previously mentioned, increases the cost, particularly when such towers are used in systems having a very large capacity.
A further solution to the channeling problem has been proposed by providing a number of inlets into the tower to obtain a more uniform inlet flow distribution within the tower. However, the flow from each of the several inlets is continuous and thus multiple channeling can still occur with a stream originating from each inlet.